1. Field of the Invention
The present invention relates generally to control systems for inverters for conversion of electric power and, more particularly, to an apparatus and method for detecting the presence of charges of appropriate magnitude and polarity on the commutating capacitors of an inverter of the type that converts an incoming DC signal to a drive current of variable current magnitude and frequency, and for providing an output signal when the appropriate charges are detected which is effective to control the generation of the drive current.
2. Prior Art
Variable frequency inverters are well known for converting an incoming DC signal from a DC source to an outgoing AC signal of variable magnitude and frequency in accordance with gating control signals provided to the inverter.
There are many types of variable frequency inverters. One of these is the controlled current inverter, which is supplied with an incoming DC current signal of variable magnitude and generates therefrom a drive current of variable magnitude and frequency in accordance with gating control signals supplied to the controlled current inverter. Such a controlled current inverter has been used in AC electric motor drive systems for generating the drive current of variable magnitude and frequency for driving an AC electric motor to produce a controllable rotation and torque.
Many variable frequency inverters, including the controlled current inverter, employ commutating capacitors to commutate automatically and sequentially the conduction controlled rectifying devices used in these inverters.
The automatic sequential commutation provided by the commutating capacitors is the result of the charges of appropriate magnitude and polarity, which are automatically built up on the commutating capacitors during the normal commutation of the conduction controlled rectifying devices in the inverter. In other words, the gating of the conduction controlled rectifying devices to the conduction state results in the build-up of charges of appropriate magnitudes and polarities on the commutating capacitors during normal operation to effect automatic commutation of the controlled rectifying devices.
Conventional variable frequency inverters employing commutating capacitors to provide automatic sequential commutation, however, exhibit several major deficiencies.
First, a charge of appropriate magnitude and polarity is not necessarily present on each of the commutating capacitors when the controlled current inverter is first started. The absence of the charges of appropriate magnitudes and polarities is due to the fact that the conduction controlled rectifying devices have not yet been gated. As stated above, these charges are caused to be built up during normal operation of the inverter.
The second major deficiency is that the charges of appropriate magnitude and polarity are bled from the commutating capacitors during conduction of the controlled rectifying devices and this bleeding becomes relatively more pronounced when the inverter is providing drive current at low frequency. This is especially critical when the inverter is providing drive current at substantially zero current and the initial charge on the capacitor is low at starting. In this case, because of the lack of sufficient charge, when the inverter is next called upon to commutate, the desired automatic sequential commutation desired from the commutating capacitors will not occur.
These and other deficiencies can prevent the inverter from producing the desired drive current of variable magnitude and frequency. This improper operation is particularly detrimental when the inverter is providing the drive current of variable magnitude and frequency to drive an AC electric motor for producing a desired rotation and torque. Failure of the motor to generate the desired rotation and torque means that the AC electric motor drive system can produce an undesirable output response.